Exocrine: secretions released through a tube or duct, e.g. salivary gland or release of enzymes from pancreas.

Endocrine: secretions released directly into the bloodstream, e.g. thyroid gland or release of hormones from pancreas.

Hormone: a chemical messenger which is produced by cells or tissues of the endocrine system, and which travels through the blood to act on target cells or organs before being broken down.

Main functions of the endocrine system-

Maintenance of homeostasis by control of temperature, pH, blood sugar levels and water balance (osmoregulation).

Works with nervous system to help body respond to stress, e.g. adrenalin release.

Controls rate of body growth.

Controls sexual development and reproduction.

Comparison of nervous and hormonal control

Property

Nervous system

Endocrine system

Nature of signal

Electrical impulse+chemical transmitter across synapse

Chemical

Size of signal

Depends on number of impulses in unit time and number of neurons stimulated, i.e. frequency modulated

Depends on concentration of hormone, i.e. amplitude modulated

Speed of signal

Very rapid (up to 120ms-1)

Usually much slower

Effect in body

Very localised as each neurone links with only one or a few cells

Much more general- hormones can influence cells in many different parts of the body

Capacity for modification

Can be modified by previous experience

Cannot be modified by previous experience

Control of digestive secretions is an example of combined nervous and hormonal control.

Nervous phase: nerve reflex in salivary glands (mouth) and gastric glands (stomach) are stimulated by the sight, smell and taste of food. When food leaves the stomach, the vagus nerve stimulates the release of bile and pancreatic juice.

Hormonal phases

gastric phase: the presence of food in the stomach stimulates the release of the hormone gastrin into the bloodstream. Gastrin acts on the gastric glands, causing the release of pepsinogen and HCl.

Intestinal phase: the acidified chyme from the stomach stimulates cells in the duodenum lining to produce secretin. Secretin causes the production of bile in the liver and NaHCO3 in the pancreas. Cholecystokin-pancreozymin (CCK-PZ) is also produced, which causes the release of bile and digestive enzymes. Enterogastrone is released as well- this reduces gastrin secretions in the stomach once food has left it.

The hypothalamus and pituitary gland are ‘partners’ in communication and control. The pituitary gland (master gland) controls the activity of most other glands but is itself controlled by the hypothalamus. The pituitary gland has both a posterior and anterior lobe- the posterior lobe is connected to the brain by the pituitary stalk, allowing nervous communication with the hypothalamus. A rich blood supply links the hypothalamus and anterior pituitary gland, allowing hormonal signals to pass between them.

Role of cyclic AMP

Hormone acts as first messenger. As it is amino-acid based (water soluble) it cannot pass directly through the lipid membrane. Instead, it must combine with a complementary receptor.

When the hormone binds to the receptor, the enzyme adenylate cyclase, (located on the inside of the membrane) is activated.

Adenylate cyclase catalyses the conversion of ATP to cyclic AMP. Cyclic AMP is the second messenger– it can cause biochemical changes within the cell. The extent of the response is determined by the concentration of cyclic AMP.

Cascade effect: a small concentration of hormone produces a disproportionately large effect as cyclic AMP amplifies the response.

Auxin: a growth hormone produced by plants such as coleoptiles (plants where a sheath surrounds the first time leaf). Auxin is constantly produced by cells at the tip of the coleoptile. Unilateral or uneven does not change the amount of auxin produced, but it does cause its redistribution. Auxin ‘moves away’ from the lighted side of the plant and accumulates on the darker side.

In shoots, more growth occurs on the concentrated side, so the shoot grows towards the light.

In roots, less growth occurs on the concentrated side, so the root grows away from the light.

Phototropism: a directional response to the stimulus of light, as directed by auxins. Shoots display positive phototropism as they move towards the light (for photosynthesis), whilst roots display negative phototropism as they move away from light (ensuring they move into the soil to obtain water and minerals). These responses have survival value for the plant, as they ensure roots and shoots move towards the resources they require.

Mechanisms in shoots directed by auxin: there are two theories.

Auxin causes the secretion of H+ ions into cell wells. The increased acidity weakens bonds in cellulose microfibrils, allowing them to slide about. As water enters the cell by osmosis, the cell is able to elongate.

Auxin triggers a change in the rate of transcription. This increases the rate of protein synthesis and thus increases growth rate.

Commercial applications of auxins

Selective weedkiller. A synthetic chemical, similar to auxin, is used on dicotyledonous weeds. The weeds taken in the weedkiller, which causes increased growth; ultimately, the plant grows so quickly that it cannot support itself and so dies.

Rooting powders- stimulates cut shoots to produce roots.

Role of gibberellins in germination: within a seed, the embryo secretes gibberellic acid (GA), which passes to the aleurone layer. GA promotes mobilisation of amino acids which form hydrolytic enzymes (e.g. amylase, protease, lipase). These enzymes digest the insoluble food stores in the endosperm so that the digested foods can be absorbed by the developing embryo and used for growth.